1. Field of the Invention
The present invention relates generally to nuclear waste storage systems and, more particularly, to a system for increasing the storage capacity of a water pool used for the storage of spent nuclear fuel rods.
2. Description of the Prior Art
Heretofore, an assortment of methods and apparatus have been developed for storage of spent nuclear fuel. They have included, inter alia, underground burial, "dry storage", and "wet storage". In underground burial, storage of the spent fuel has involved sealing of the fuel in containers and then burying the sealed containers at a predetermined distance beneath the ground surface. Such a system is disclosed in U.S. Pat. No. 3,935,467. Dry storage generally involves the storage of spent fuel in sealed canisters which are placed in racks, wherein the exteriors of the containers are cooled by convention by way of the flow of a cooling gas such as air, in contact therewith. Examples of such a spent nuclear fuel storage system are provided in U.S. Pat. Nos. 4,525,324, and 4,299,659 as well as West German Offenlegungsschrift No. 31 01 540 and European Patent No. 54,968.
Perhaps most commonly, however, spent nuclear fuel rod assemblies, once removed from the nuclear reactor core, are stored in underwater or wet storage areas in specifically designed storage racks for sufficient periods of time to permit the residual reactivity of the nuclear fuel contained therein to dissipate, or until the spent fuel rod assemblies are withdrawn from the pool for transportation to a spent fuel recovery facility. The fuel rods are metal pipes which are filled with nuclear fuel material and are typically 0.4-0.6 inches in diameter and 8 to 15 feet in length. Groups of 64, 128, or more such rods are confined within a fuel rod assembly while they are in a nuclear reactor and subsequently while they are stored in the underwater storage area. It is a normal practice to provide empty spaces in the storage rack between spent fuel rod assemblies to preclude any unintended temperature increase within the underwater storage area. When an underwater storage area has been filled with spent fuel rod assemblies in such a manner, there is no space remaining within the storage area for additional spent fuel rod assemblies.
Increasing the density of the material stored in the underwater storage area allows a greater amount of nuclear waste material to be stored in the limited available space of the underwater storage area. To accomplish this end, it has been proposed to withdraw the spent fuel rods from the assembly in which they have a normal spacing and to compact the spent fuel rods in a canister in which the fuel rods are compacted in tighter spaces. The more compact spent nuclear fuel rods do not present the same tendency for temperature increase and can be stored in adjacent and contiguous spaces in a storage rack, thereby increasing the storage capacity of an existing storage rack three-fold.
The primary approaches to support spent fuel rod assemblies in an underwater storage area or "pool" can be classified as floor-supported and wall-supported storage systems. In floor-supported systems, such as those described in U.S. Pat. Nos. 4,889,681; 4,666,660; 4,336,103; 4,069,923 and 4,029,968, the entire weight, or a portion thereof, of the fuel storage rack and the stored spent fuel assemblies is borne upon the floor of the storage pool.
A major factor limiting the increase of the storage capacity of floor-supported storage systems, however, is that the loading capacity of the floor of the storage area may be exceeded by such increased storage. Additionally, because it is essential to maintain the spent fuel rods under at least 10 feet of biological shield water at all times, including those times when the spent fuel rods are being transferred from their fuel rod assemblies into a compacted fuel rod canister. The great volume of water further increases the loading exerted on the floor. The floor loading, therefore, is the weight of the volume of water in the storage area, the storage rack and the spent fuel rod canisters contained therein.
In order to reduce the weight exerted by the fuel rack assemblies on the pool floor, U.S. Pat. Nos. 4,889,681 and 4,666,660 have proposed attaching buoyant chamber means to the top of the spent fuel rack assemblies.
In wall-supported storage systems, such as those described in U.S. Pat. No. 4,287,426 and West German Patent No. 29 43 455, the spent fuel rack assemblies are generally suspended from upper portions of the walls of the pool whereby the assemblies are spaced some distance from the walls and the floor of the pool. Particular advantages to such systems include: 1) their inherent resistance to the effects of seismic shock and the potentially damaging ground displacements attendant thereto by virtue of the system being suspended a predetermined distance from the pool floor and walls, 2) their capacity to distribute to the weight of the fuel rack assemblies through the reinforced walls of the pool and thereby over a greater area of the surrounding earth, and 3) their avoidance of the pool floor as a support surface. However, the vertical orientation of the fuel canisters in the fuel racks of these wall-supported systems has prevented these systems from being safely used with other fuel storage rack systems in the same storage pool in order to increase the storage efficiency of the volume of the pool.
Except for U.S. Pat. No. 4,029,968, none of the aforementioned patent disclosures has provided any means or methods for increasing the storage efficiency of the volume of a spent nuclear fuel storage pool. In that patent, it was proposed to vertically stack a plurality of interconnected spent fuel storage racks, each of which support clusters of vertically oriented fuel storage canisters, atop one another with the bottom rack being supported by the pool floor. While such disclosure is seen as an advancement in the concept of increasing the storage efficiency of the volume of a spent fuel storage pool, it is seen as a design which is potentially harmful to the structure of the pool, particularly the floor thereof, because the concentrated weight of the plurality of vertically-stacked spent fuel rack assemblies is applied directly to the pool floor, thereby providing a greatly increased likelihood of exceeding the loading capacity of the floor. Moreover, the depth of pool required for safe storage of the vertically stacked racks usually greatly surpassed the depth of existing storage pools at most nuclear facilities. Therefore, in order to accommodate such a storage system, new, especially deep, and somewhat unconventional pools would have to be constructed at most facilities, hence involving potentially immense capital investments in new plant constructions as well as retrofitting existing facilities.
An advantage exists, therefore, for a spent nuclear fuel storage rack system which increases the storage efficiency of the volume of spent nuclear fuel storage pools of conventional dimensions and simultaneously does not increase the likelihood of exceeding the loading capacity of the structure forming the pool.
It is therefore an object of the present invention to provide a spent nuclear fuel storage rack system which increases the storage efficiency of the volume of spent nuclear fuel storage pools of conventional dimensions and simultaneously does not increase the likelihood of exceeding the loading capacity of the structure forming the pool.
It is a further object of the present invention to provide either a stationary or mobile spent fuel rack assembly which is adapted to be pivotally or non-pivotally suspended from the walls of a spent nuclear fuel storage pool above and disconnected from conventional floor-supported spent fuel rack assemblies contained therein.
It is a further object of the present invention to provide a novel spent nuclear fuel storage canister construction.
It is a further object of the present invention to provide a novel apparatus for handling of spent fuel storage canisters.
Still other objects and advantages will become apparent in light of the attached drawings and written description of the invention presented hereinbelow.